In particular, the school systems that are around such homes. Location, Location, Location.
Secondly: those houses have tenants. They're not sitting empty.
Thirdly, for the young couple. Being tied to your parents financially basically makes them a slave-owner of you. When the parents drop by or use the yard to store project-cars, or refuse to fix issues with the house... the young couple is not in a position to fix those issues themselves without pissing off the parents. (Its not their house after all). So getting their own house leads to a better relationship between child and parent anyway, and should be preferred.